Critical to the industrial utilization of organic acids which are generated by fermentation of carbohydrate-containing substrates by various microorganisms, or which come from synthesis solutions, are the economics and efficiency of the purification, concentration and the crystallization.
WO 2011/160760 A1 discloses a method for removing, recovering and purifying dicarboxylic acids. It describes removal of the biomass from the fermentation broth in two consecutive filtration steps. The dicarboxylic acid solution is subsequently removed from the biomass-free fermentation broth by means of Simulated Moving Bed (SMB) chromatography. After fine purification through a membrane system, activated carbon filter and/or cation exchanger and/or anion exchanger, the purified dicarboxylic acid solution undergoes a multistage evaporative operation for concentration, followed by an evaporative crystallization. In the crystallization, there is slow cooling of the concentrated dicarboxylic acid in steps of 3° C. to 8° C./min, preferably 3° C. to 5° C./min, and, for obtaining high-purity dicarboxylic acid, in steps of 1° C. to 5° C./h. For the recovery of high-purity crystals, the evaporative concentration operation and the crystallization are repeated a number of times. For this purpose the crystals are dissolved in demineralized water and/or vapor condensate. The residual crystallization solution arising after crystallization, through separation of the crystals, and referred to hereinafter as mother liquor, is returned before the evaporative concentration. This method can be implemented for dicarboxylic acids selected from the group of fumaric acid, maleic acid, adipic acid, itaconic acid, benzoic acid and others, especially succinic acid.
DE 38 27 455 C2 describes a method for separation. In the case of this method, a single-stage or multistage fractional cooling crystallization is utilized, and the mixture, which fills the crystallizer completely, is set into vibration by means of forced oscillations in the course of solidification. After the end of the solidification, the remaining mixture is removed and the crystal layer is melted off in a plurality of temperature stages, a melt of identical composition being introduced into the crystallizer for the melting-off operation. The fractions recovered accordingly are collected individually. The apparatus for this method consists of a crystallizer with heat exchanger and an associated temperature circuit. The heat exchanger is preceded by a compensating vessel which has a separate conditioning circuit, and a liquid-phase vessel is connected at the bottom of the heat exchanger. Beneath the liquid phase there is an oscillating device which sets the mixture in the crystallizer into turbulent vibrations during solidification.
DE 600 14 047 T2 describes a method in which a dilute lactic acid concentrate is crystallized by direct cooling in one or more melt crystallizers, or is crystallized by means of one or more cooling crystallizers and/or evaporative crystallizers and/or one or more adiabatic crystallizers. In this case, seed crystals are used for the method of crystallization.
DE 600 28 806 T2 presents a method wherein selected salts, such as potassium, magnesium and ammonium salts, nitrates, phosphates, sulfates and organic salts, are obtained from an aqueous solution, such as a liquid agricultural or fermentation byproduct or a derivative thereof, a waste stream from an acid-catalyzed chemical process, an aqueous solution from gas scrubbing of off-gases from an agricultural stall for the keeping of cattle, or an aqueous solution from the processing of cattle manure. These salts are crystallized by freeze crystallization, with ice and crystallized material being separated from one another by exploiting their density properties and/or their difference in particle size.
DE 10 2012 105 128 A1 discloses a method by means of cooling crystallization for purifying or processing long-chain dicarboxylic acids or one of their salts. In this case the crude material is first acidified to a pH of 1 to 2.5 at a temperature from 60° C. to 100° C., and the precipitating crude product is collected, and dissolved by addition of an alkaline solution (5M NaOH), with a pH of 1 to 2.5 being retained. Subsequently the dicarboxylic acid is heated at 80° C. for an hour and melted and insoluble material is removed by filtering, the filter medium consisting one kind of several kinds of gauze, nylon membrane, ceramic foil, metal foil and glass fiber membrane. The dicarboxylic acid is decolorized using 0.2 wt % of activated carbon or 0.5 wt % of silica and is thereafter acidified again to a pH of 1 to 2.5. After centrifugation, the resulting precipitate of the dicarboxylic acid is collected and is washed one to ten times with water until neutral (pH 6.5 to 7) at a temperature of 85° C. This is followed by heating at 60° C. to 100° C., filtration, further washing and subsequent drying, producing a filtercake of the dicarboxylic acid, which is in turn suspended under high pressure in water and heated to >100° C., the temperature being maintained for 20 minutes above the melting point. Subsequently the temperature is slowly lowered to room temperature at 10° C. to 15° C./h, followed by further filtration of the dicarboxylic acid, in order to thus obtain the dicarboxylic acid crystals. In the method, alternatively, it is possible to omit the first heating and melting of the dicarboxylic acid, which takes place at 80° C. for 1 hour, and/or to omit the first acidification to a pH of 1 to 2.5.
A disadvantage of all of these methods is that the practical implementation of the methods entails considerable technical and energy expenditure and complexity.